Gosh, they're actually two different processes though similar. When you marinate something, you soak it in brine before actually cooking it not much later. So you can soak a rack of lamb in a brine that you make up for a couple of hours and then roast it in the oven. When you pickle something you're actually preserving it in a brine for a longer period of time, and you don't necessarily cook the item after it. So gherkins and other vegetables are often pickled to preserve them and eaten as they are.

Basically "to marinate" means to soak some food in seasoned food before cooking, whereas "to pickle is the process to store some food in a jar full of vinegar or salt and water for a long time. The marination is done to add flavour and taste to your cooking and the pickling is done to preserve the food for later use and it also protects it from bacteria.

When we marinate a food, it soaks up the flavor (i.e- good stakes are marinated before being grilled). When we pickle something, it is to preserve a food. Usually vinegars or salted water is used. It adds some tang to the taste. Does this help?